While the number of new HIV infections has been relatively stable since late 1990s, the face of the infection has changed. Half of all new infections now occur among youth and substantial disparities exist based on sexual orientation. Recent data from the National HIV/AIDS Reporting System indicate that male-to-male sexual contact accounts for 76-79% of HIV diagnoses among 16-19 year old males, although less is known about the actual prevalence of HIV and related health issues in this population. Disparities also exist in preparedness to prevent HIV among young men who have sex with men (YMSM), with no proven intervention for adolescents in this vulnerable group. The epidemic of HIV among YMSM does not exist in a vacuum. Instead, YMSM experience multiple psychosocial health disparities, including elevated rates of illicit drug use, exposure to violence, and mental health problems such as suicidality. Recent evidence indicates that these health issues represent intertwined epidemics that each contribute to HIV risk among YMSM, a situation referred to as a syndemic. Building on our own research with YMSM, as well as other high risk youth, we are proposing to investigate a syndemic of psychosocial health issues linked to HIV among YMSM ages 16-20. This syndemic includes HIV risk, drug use, internalizing mental health problems, and violence exposure. The overarching goals of this study are twofold: 1) to provide much-needed epidemiological data on the prevalence of HIV and related health issues in order to inform public health priorities; and 2) to collect vital information on risk and protective factors to inform the development of an intervention targeting this vulnerable population. We will accomplish these goals with three primary specific aims: (1) Ascertain the prevalence and clustering of a syndemic of biopsychosocial health problems among YMSM. We will: (a) Recruit a representative sample of 450 ethnically diverse YMSM using Respondent Driven Sampling (RDS); (b) Assess the syndemic components using state-of-the-art approaches; (c) Use the RDS model to calculate unbiased estimators of the prevalence of syndemic components; (d) Test for the clustering of syndemic components. (2) Characterize multiple trajectories of syndemic development by following our YMSM sample longitudinally over a two year period. We will: (a) Assess youth every 6 months during the transition from late adolescence to emerging adulthood, to explore the natural history of syndemic development. (b) Use growth-mixture modeling to characterize groups of youth with different trajectory classes of syndemic development in order to allow for early identification of youth most at risk of HIV acquisition. (3) Determine the importance of YMSM-specific factors in syndemic development so as to guide the development of culturally-relevant HIV prevention interventions for our target population. We will: (a) Test the value of YMSM-specific factors as predictors of trajectory class; (b) Test the incremental value of our theoretical syndemic model for predicting HIV-related sexual risk relative to the IMB model.